Conventional smoke detection units -can be part of a scattered light smoke alarm in a closed or open construction. In a closed construction such a scattered light smoke alarm comprises an optical measuring chamber which is permeable for smoke to be detected but is screened against direct environmental light. Such a measuring chamber is often also designated as a labyrinth. On the other hand, scattered light smoke alarms of with an open construction comprise a detection chamber located outside of the alarm housing in the open. Finally, such smoke detection units can be part of an extinction smoke alarm in which the attenuation of light by present smoke is detected and evaluated by the photodetector.
It is known that light-emitting diodes age during operation and in the course of time have a reduced light performance. In contrast to the above, photosensors such as, e.g., photodiodes have comparatively small, negligible aging phenomena. Consequently, the light performance is to be monitored in order to emit a warning notice in case of a light performance which is too low for the detection of smoke. Alternatively, the control performance or the pulse duration for the control of the light-emitting diode can be increased in order to correct the reduced light performance and/or the reduced emitted light current. To this end the use of a separate photosensor is known which preferably receives direct light from the light-emitting diode and is arranged opposite the light-emitting diode. The outputting of the warning alarm then takes place as a function of the photodetector signal and/or a light performance compensation takes place via the path of the electrical control of the light-emitting diode.
Furthermore, it is known that all optoelectronic structural elements in the smoke detection unit become contaminated in time. That means that the optical sending power of the light-emitting diode and the optical detection sensitivity of the photodetector for the detection of smoke and of the separate photodetector for the monitoring of the light performance and/or light current decrease as the contamination increases and independently of the ageing of these optoelectronic structural elements.
Finally, light-emitting diodes, in particular one-, two- or multi-color light-emitting diodes are known from the prior art which are one-piece structural elements. Such light-emitting diodes comprise a LED housing, at least one LED chip arranged in it for the particular “color” and comprise the connection contacts contacting the particular LED chip and running out of the LED housing.